The present invention relates to a new and improved construction of smoke detector containing a radiation source operated in a pulse-like or intermittent mode.
In its more particular aspects the smoke detector of the present development is of the type comprising a pulse-operated radiation source emitting focussed radiation into a region freely accessible to environmental air, a radiation receiver arranged in the region of said radiation or radiation pulses, and an input amplifier series connected to the radiation receiver and generating output pulses proportional to the intensity of the radiation impinging upon the radiation receiver. There are also provided an evaluation circuit comprising a reference voltage generator for generating a reference voltage for comparison with the output pulses, and an alarm stage defining an alarm threshold for triggering an alarm signal when the output pulses have been attenuated below a predetermined value of the alarm threshold for more than a first predetermined period of time. There is further provided a disturbance circuit or stage defining a disturbance threshold and triggering a disturbance signal when the output pulses have been more rapidly attenuated than during triggering the alarm signal, and wherein the disturbance threshold is lower than the alarm threshold.
A smoke detector of the aforementioned type is known, for example, from German Patent Publication No. 2,822,547. In the smoke detector described therein the radiation source and the radiation receiver are accommodated in two different housings, as is usual in such so-called "line extinction alarms or detectors". These housings are mounted at the walls of the room or area to be monitored at a distance from one another depending on the requisite location of use. A fixed alarm threshold is predetermined which, however, depending upon the different distances between the radiation source and the radiation receiver corresponds to totally different smoke densities.